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04/07/2003 07:00:00

Science begins for LIGO in questto detect gravitational waves

Armed with one of the most advanced scientific instruments of all time, physicists are now watching the universe intently for the first evidence of gravitational waves. First predicted by Albert Einstein in 1916 as a consequence of the general theory of relativity, gravitational waves have never been detected directly.

In Einstein's theory, alterations in the shape of concentrations of mass (or energy) have the effect of warping space-time, thereby causing distortions that propagate through the universe at the speed of light. A new generation of detectors, led by the Laser Interferometer Gravitational-Wave Observatory (LIGO), is coming into operation and promises sensitivities that will be capable of detecting a variety of catastrophic events, such as the gravitational collapse of stars or the coalescence of compact binary systems.

The commissioning of LIGO and improvements in the sensitivity are coming very rapidly, as the final interferometer systems are implemented and the limiting noise sources are uncovered and mitigated. In fact, the commissioning has made such rapid progress that LIGO is already capable of performing some of the most sensitive searches ever undertaken for gravitational waves. A similar device in Hannover, Germany (a German–U.K. collaboration known as GEO) is also getting underway, and these instruments are being used together as the initial steps in building a worldwide network of gravitational-wave detectors.

The first data was taken during a 17-day data run in September 2002. That data has now been analyzed for the presence of gravitational waves, and results are being presented at the American Physical Society meeting in Philadelphia. No sources have yet been detected, but new limits on gravitational radiation from such sources as binary neutron star inspirals, selected pulsars in our galaxy and background radiation from the early universe, are reported.

Realistically, detections are not expected at the present sensitivities. A second data run is now underway with significantly better sensitivity, and further improvements are expected over the next couple of years.

As the initial LIGO interferometers start to put new limits on gravitational-wave signals, the LIGO Lab, the LIGO Scientific Collaboration, and international partners are proposing an advanced LIGO to improve the sensitivity by more than a factor of 10 beyond the goals of the present instrument. It is anticipated that this new instrument may see gravitational-wave sources as often as daily, with excellent signal strengths, allowing details of the waveforms to be read off and compared with theories of neutron stars, black holes, and other highly relativistic objects. The improvement of sensitivity will allow the one-year planned observation time of the initial LIGO to be equaled in a matter of hours. The National Science Foundation has supported LIGO, and collaboration between Caltech and MIT were responsible for its construction. A scientific community of more than 400 scientists from around the world are now involved in research at LIGO.